Quiescent current control schemes reduce the variation in quiescent current in Class AB output stages. Quiescent current control schemes can be divided into two types: 1) Open Loop and 2) Feed back types. This patent application describes a feedback type of quiescent current control.
The analog line driver on any line interface unit tends to use a lot of power relative to other parts of an integrated circuit, and is therefore a prime target for power reduction. Multiple design techniques have been employed in the past to achieve tighter control of the quiescent current of the amplifier. However, achieving the desired control while reducing the unintended effects (such as distortion) on the circuit is an ongoing challenge.
Quiescent current control schemes reduce the variation in quiescent current in Class AB output stages. The different approaches can be divided into basic types as follows: Type 1—relies on open loop arrangement to control quiescent current, and does not use feedback; Type 2—uses feedback to suppress variations in the quiescent current. Within the Type 1 (open loop) arrangements, sometimes non-linear elements are used to enhance quiescent current control at the cost of increased distortion. Within Type 2 (feedback arrangements), one must be careful to feed back the correct combination of the output stage signals to achieve close control of quiescent current without increasing distortion.
Each of the types of quiescent current control identified above reduces quiescent current variations, but suffers from one or more problems including not ensuring solid quiescent current over process, voltage, and temperature (PVT) process variations, causing crossover distortion or gain anomalies around the crossover region, and/or not maintaining minimum current in the non-active output device when the active device conducts heavily.
Circuits developed by Seevinck are of Type 2 (feedback), and work well in bipolar processes, while not suffering gain reduction in the crossover region. Seevinck accomplishes this by feeding back a current that is proportional to the minimum of the currents in the output devices. However, application of this approach to CMOS (complementary metal-oxide semiconductor) processes requires the use of bipolar transistors that are made within the CMOS process. Such bipolar transistors tend to be not very fast relative to the MOS transistors, and are typically not well-matched. The lack of speed in the CMOS-bipolar transistors requires them to be operated with higher currents than is desirable. The lack of matching limits the precision of the quiescent current control. The relatively small signals involved in the operation of the bipolar-based minimum circuits also limit the accuracy of the quiescent current control in CMOS amplifiers.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.